The invention relates to a method of transmitting encoded data defining a graphic image in the form of a rectangular region within an active video area. The invention also relates to a method of receiving said graphic images, and a transmitter and receiver arranged to carry out said methods.
A prior art of transmitting graphic images is generally known by the name xe2x80x9cteletextxe2x80x9d. The World System Teletext standard has been adopted in may countries for the simultaneous transmission of textual and graphic information with a conventional analog television signal. The teletext system is particularly used for the transmission of subtitles associated with a television program.
Meanwhile, the MPEG2 (ISO/IEC 13818) digital television standard will be applied worldwide for the broadcast of digital television program to the end-user by satellite, cable, terrestrial networks, and by packaged media such as tape or disc. A multitude of integrated circuits for MPEG2 decompression and demultiplexing are on the market and will be applied in receiver decoders for the purpose of receiving and decoding of broadcast television programs or decoding of packaged (multi-)media.
The MPEG2 compression and multiplexing technology will provide the end-user a very large choice in programs (typically in the order of hundreds), which will require an easy-to-use xe2x80x9cman-machine interfacexe2x80x9d. It is expected that the end-user will associate xe2x80x9cdigitalxe2x80x9d with high quality. High quality must therefore be manifest in all aspects of the service that will be provided. Typical of satellite broadcast is the large footprint of its reception area, which usually encompasses multiple language areas. Many of the television program to be broadcast will therefore also be targeted at multiple language areas, which requires multi-language subtitling capabilities. The attractiveness and readability of subtitles will have a major in one end-user""s willingness to watch a television program in a language spoken other than his or her own.
The prior art teletext system is very limited in quality and performance. It is a character-based transmission system, in which coded data is transmitted to define which characters are to be displayed. The appearance of the subtitles (in terms of character font, spacing, graphic capabilities, colours) is fixed in hardware. Further, the present teletext system is not able to support complex character sets such as used in the People""s Rpublic of China and Japan. The character-based system would require very expensive and complex character generators in case of supporting multiple character sets, or in case of a single but very complex character set. The prior art teletext system thus provides only a basic solution for multi-language subtitling.
It is an object of the invention to overcome the above mentioned disadvantages of the prior art system. It is a further object to provide a universal solution for multi-lingual subtitling, in particular, for an MPEG2 environment.
The method according to the invention is characterized in that the pixels forming said region are individually defined by the encoded data, the size and position of said region being included in the encoded data. The method renders it possible for the program provides to control the appearance of the graphic images pixel-wise. The xe2x80x9cRegion based Graphicxe2x80x9d system provides bitmapped rather than character coded regions overlaying the video signal. There are no restrictions as regards character size and character font, and there is a capability to overlay other graphics than text, e.g. the program provider""s logo.
In an embodiment, the enclosed data further includes a time stamp representing the time at which said region is to be displayed. This allows multilingual subtitles to be transmitted in advance and to be displayed in exact synchronism with the video signal. In the prior art teletext system, the subtitles in the selected language are immediately displayed when received. More or less precise synchronization is possible only if all subtitles in the plurality of languages are transmitted within a restricted period of time.
Subsequent encoded data for a given region may include a visibility code to disable the display of said region. This allows for effectively erasing a graphic region.
In further embodiments of the invention, the pixels of a region are encoded as entries of a colour-look-up-table. Map tables are provided for mapping n-bit pixel codes to a colour-look-up-table having more than 2n entries.
In an particularly advantageous embodiment, the encoded data is accommodated in a private transport stream according to the MPEG2 or a similar standard. Packetizing the encoded data in PES packets of an MPEG2 private transport stream has the advantage of easy synchronizing the bitmaps to an associated MPEG video system.